Polypropylene resin is a kind of general resin, which is easy to process and has excellent properties for the cost, so that it has wide spectrum of use, precisely it takes the place of conventional materials such as glass, wood, paper, metal, etc, and also can be applied to the field of other plastic products even including engineering plastic. However, homopolypropylene resin has low impact strength even though it has high strength, and thus it is limited in use. To improve impact strength of such polypropylene, propylene-ethylene block copolymer is prepared by melt intercalating or adding ethylene-propylene copolymer or EPDM (ethylene propylene diene copolymer) to homopolypropylene in a reactor. Propylene-ethylene block copolymer has excellent impact strength, so that it can be widely applied in injection molded products such as automobile parts, house appliances, parts for industrial fields, general goods, and packaging containers, packaging films and sheets. However, the molded product produced from propylene-ethylene block copolymer has very poor transparence even though it has high impact resistance, so that it is limited in use for molded articles for food packaging, blow molding materials, films, sheets, etc. So, random copolymer produced by simultaneous polymerization of propylene and a small amount of comonomer (mainly ethylene or butene) in the same reactor is used. Random copolymer has low working temperature, for example low heat-sealing temperature, and high transparence. Therefore, it can be used for such products requiring high transparence as food packaging containers, films, sheets, and blow molding materials, etc, but because of its low impact resistance, which is as low as that of homopolypropylene, it is difficult to be used at low temperature, for example for freezer or refrigerator containers, etc.
To have advantages of both propylene-ethylene block copolymer and random copolymer, which are excellent impact resistance and excellent transparence, Korean Patent No. 156560 describes a method for regulating the ratio of intrinsic viscosity of ethylene-propylene random copolymer and elastic ethylene-propylene copolymer. The composition obtained by this method, however, did not show any improvement in transparence and still had low impact strength at low temperature.
Korean Patent No. 10-0500497 describes a method for regulating the ratio of intrinsic viscosity and melt index of olefin copolymer and monopolymer and ethylene-propylene copolymer. However, the resin composition obtained by this method did not show satisfactory improvement in transparence, so that it is not proper to be used for thick products including injection products.
Korean Patent No. 10-0361550 describes a method for preparing a resin composition for extrusion blow molding, in which ethylene-propylene random copolymer is melt-mixed with ethylene-alpha olefin copolymer elastomer prepared by using a metallocene catalyst. According to this method, it is difficult to distribute the elastomer on the ethylene-propylene random copolymer, resulting in the product with unsatisfactory transparence and low impact strength at low temperature.
Korean Patent No. 10-0484603 describes a resin composition composed of ethylene-propylene random copolymer and elastic ethylene-propylene copolymer with providing intrinsic viscosity value of the ethylene-propylene elastic copolymer. But, if absolute viscosity of ethylene-propylene random copolymer is lower than that of elastic ethylene-propylene copolymer, transparence is decreased. Thus, this method is limited in preparing a composition having high transparence and high impact resistance.
Korean Patent Publication No. 2006-0116557 describes a resin composition comprising ethylene-propylene random copolymer composed of ethylene-propylene or propylene-butene-1 random copolymer and elastic ethylene-propylene copolymer, and a method for increasing transparence and impact resistance by regulating the ratio of ethylene/propylene in ethylene-propylene elastic copolymer and the ratio of melt index of ethylene-propylene elastic copolymer. However, the composition prepared by this method still has low impact strength at low temperature.